From Wikipedia, the free encyclopedia

A period 3 element is one of the chemical
elements in the third row (or period) of
the periodic table
of the chemical elements. The periodic table is laid out in
rows to illustrate recurring (periodic) trends in the chemical
behaviour of the elements as their atomic number increases: a new
row is begun when chemical behaviour begins to repeat, meaning that
elements with similar behavior fall into the same vertical columns.
The third period contains eight elements: sodium, magnesium,
aluminium, silicon, phosphorus, sulfur, chlorine, and argon. The
first two, sodium and magnesium, are members of the s-block of the periodic table,
while the others are members of the p-block.